Thin film resistor anodizing and monitoring circuit

ABSTRACT

CIRCUIT. THE TIMING CIRCUIT IS INITATED WHEN THE ANODIZING VOLTAGE REACHES A PREDETERMINED MAGNITUDE FOR TERMINATING THE ANODIZING IS THE RESISTANCE FAILS TO INCREASE TO THE PREDETERMINED VALUE WITHIN A SET PERIOD OF TIME.   AN ANODIZING AND MOINTORING CIRCUIT ALTERNATIVELY APPLIES AN ANODIZING VOLTAGE TO A THIN FILM RESISTOR AND MOMITORS THE RESISTANCE TO INCREASE THE RESISTANCE TO A DESIRED VALUE. THE ANODIZING VOLTAGE IS CONTINUALLY INCREASED DURING THE ANODIZING AND MONITORNG UNTIL THE ANODIZING IS TERMINATED BY THE RESISTANCE INCREASING TO A PREDETERMINED VALUE OR BY THE OPERATION OF A TIMING

June 22, 1971 w 5 BURNS, JR" ETAL 3,586,618

THIN FILM RESISTOR ANODIZING AND MONITORING CIRCUIT 2 Sheets-Sheet l Filed Feb.

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THIN FILM RESISTOR ANODIZING AND MONITORING CIRCUIT Filed Feb. 5, 1968 2 Sheets-Sheet 2 3,586,618 Patented June 22., 1971 United States Patent Ofl-ice 3,586,618 THIN FILM RESISTOR ANODIZING AND MONITORING CIRCUIT William S. Burns, Jr., Shreveport, La., and James L. Owens, Clemmons, and George A. Patton, Pfaiftown, N.C., assignors to Western Electric Company, Incorporated, New York, N.Y.- Filed Feb. 5, 1968, Ser. No. 703,104

Int. Cl. B01k 3/00 Us :1. 204-228 3 Claims 'ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION (1) Field of the invention A thin film resistor is manufactured by depositing a thin metallic film on a nonconductive substrate. Since it is difiicult to accurately control the thickness of the metal film the film is deposited with a thickness having substantially less than the desired resistance. The film is then anodized to reduce the thickness of the conductive portion of the film until it reaches the desired value of resistance. I

(2) Description of the prior art In the prior art, there are many circuits which sequentially anodize a thin film resistor and monitor the resistance repetitively until the resistance has reached a predetermined value. However, in these prior art circuits, a constant anodizing voltage is applied to the resistance. When a low anodizing voltage is used, the anodizing requires a long time duration. A high anodizing voltage ruins the resistor by applying too much current thin film during the initial phase of the anodzing.

During the anodizing, some resistors develop inferior qualities due to impurities, etc., and require extremely long anodizing durations to reach the desired resistance. In the prior art, an operator observed the anodizing, and when a resistor fails to increase to the desired value in a short period of time, the operator manually terminated the anodizing of the inferior resistor.

' SUMMARY OF INVENTION The invention relates to a circuit for sequentially anodizing and monitoring the resistance of a thin film resistor until the resistance hasreached a predetermined value.

a predetermined value.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 show a circuit for sequentially anodizing and monitoring the value of the resistance.

FIG. 3 shows how the circuits of FIGS. 1 and 2 are connected.

DETAILED DESCRIPTION Referring to FIG. 1, there are shown a plurality of thin film resistors 10, 11 and 12 which are immersed in an electrolyte 13. An electrode 14 is immersed in the electrolyte 13 for completing an electrical path for a current through the electrolyte 13 to the resistors 10-12. The leads connected to the resistors 10-12 are insulated from the electrolyte 13, for example, by a non-conductive grease coated on the leads. Another way of supplying an anodizing current to the thin film resistor is to spread a viscous electrolyte on the resistor film and engage the electrode 14 with the viscous electrolyte.

A stepping switch 17 connects one end of a selected resistor to a corner 22 of a DC. resistance bridge circuit 20. The other ends of the resistors 10-12 are connected to another corner 23 of the bridge 20 to complete one arm of the bridge circuit 20. Resistors 19, 25 and 26 make up the other arms to the bridge circuit 20.

Referring to FIG. 2, there is shown a relay 27 which is intermittently operated by a pulse generating circuit 28. A contact arm 27a (FIG. 1) of the relay 27 alternately engages contacts 27b and 270 to alternatel connect the corner 22 of the bridge circuit 20 to the input of a DC. amplifier 29 and to a resistor 31 which is connected by lead 35 and contacts 41a to a slider 32 of a potentiometer 33. A contact arm 27d of the relay 27 alternately engages a contact 27 and 27:2 to alternately connect ground to anode 14 and a corner 24 of the bridge circuit 20. When contact arm 27a engages contact 27 c and contact arm 27d engages contact 27], an anodizing current passes from a power supply 34 connected across the potentiometer 33 through the slider 32, the contacts 41a, the conductor 35, the resistor 31, contact 27c, contact arm 27a, selector switch 17, resistor 10, electrolyte 13, anode 14, contact 27 and contact arm 27d to ground. When the contact arm 27a engages contact 27b and contact arm 27d engages contact 27c, the output corners 22 and 24 of the bridge circuit 20 are connected across the input of the amplifier 29. The values of the resistors 19, 25 and 26, and the polarity of DC. voltage applied to corners 21 and 23 of the bridge circuit 20 are chosen such that the output voltage on corners 22 and 24 is a negative voltage until the resistance 10 reaches a predetermined value at which time the voltage applied to the input of the amplifier 29 becomes positive.

Referring to FIG. 2, the slider 32 of potentiometer 33 is advanced and retracted by a reversible DC. motor 37 which is mechanically connected to the slider 32 by a firction clutch 38. When a reversing switch 45 is unoperated, a DC. voltage of a first polarity is applied to the terminals of the motor 37 by the reversing switch 45. The voltage to the reversing switch is supplied through a speed control 46, a contact 51b, and a contact arm 51a connected to a DC. voltage source to advance the slider 32 at a rate determined by the speed control 46. When the reversing switch 45 is operated, the 11C. source is connected through conductor 56 to the opposite terminals of the motor 37 to rapidly retract the slider 32.

Two series connected resistors 47 and 48 are connected from the slider 32 of the potentiometer 33 to ground with the junction of the resistors 47 and 48 connected to the input of a multi-stage transistor amplifier 49. The voltage on the slider 32 increases from a first low voltage to a second voltage as the slider 32 advances. A relay 53 in series with the amplifier 49 is operated when the voltage on the slider 32 reaches a second voltage. The slider 32 continues to advance until it reaches its end of travel whereupon the friction clutch 38 slips. Holding current through the amplifier 49 and the relay 53 maintains the relay 53 actuated until the motor 37 retracts the slider 32 and the voltage on the slider 32 has returned to the first voltage. The increasing anodizing voltage from the slider 32 provides for a short time period for anodizing without applying an initial high voltage to the thin film resistor to ruin the resistor.

Referring back to FIG. 1, when the input of the amplifier 29 becomes positive, an output signal of the amplifier 29 is applied to a control electrode of a silicon controlled rectifier 39 to initiate conduction through the rectifier 39. If contacts 53a are closed by actuation of relay 53 (FIG. 2), relays 41 and 42 are actuated by the conduction through the rectifier 39. Contacts 41a of relay 41 are opened when relay 41 is actuated to terminate the anodizing current through the electrolyte 13. Contact arm 41b disengages contact 410 and engages contact 41d to deenergize lamp '60 and energize lamp 61 to indicate that the resistor has been anodized to the desired value.

Relay 42 is connected in series with an RC network 43 such that the relay 42 is only momentarily actuated when conduction is initiated through rectifier 39. Contacts 42a of the relay 42 are closed by the actuation of the relay 42 to advance the stepping switch 17 to connect the resistance 11 to the corner 22 of the bridge circuit 20. Contact arm 42b of relay 42 disengages contact 42d and engages contact 42c to actuate relay 51. Contacts 51d of relay 51 are closed upon actution of relay 51 to complete a holding circuit for relay 51 which remains actuated until contacts 53a of relay 53 are opened. With relay 51 actuated, contact arm 51a (FIG. 2) of relay 51 engages contact 51c for actuating relay 58 to operate double-pole, double-throw contact arms 58a and 58b in the reversing switch 45 to reverse the motor 37 and rapidly retract the slider 32.

Closing of contacts 53a also applies a voltage to a time delay relay 55 (FIG. 1) until relay 42 is actuated to disengage contact arm 42b from contact 42d in series with relay 55. If the resistor 10 fails to increase to the proper value within a predetermined time after the voltage on the slide 32 has reached the second voltage, the relay 55 is actuated to close contacts 55a to bypass the rectifier 39 and terminate the anodizing. Contacts 55b of relay 55 are closed to energize an indicative device 63. Contacts 550, in series with contact arm 41b, are opened to extinguish lamp 60 and prevent energization of lamp 61. An operator alerted by the indicating device 63 may determine the defective resistor by looking at an indicator 64 which indicates the position of the stepping switch 17.

OPERATION Referring to FIGS. 1 and 2, initially, the relays 41, 42, 51, 53, 55 and 58 are unactuated. The resistor 10 is connected by the stepping switch 17 between corners 22 and 23 of the bridge circuit 20. The relay 27 is alternately actuated and deactuated by the pulse generating ciricuit 28. Contact arms 27a and 27d are alternately (1) connecting an anodizing voltage to the corner 22 and the resistor 10 with respect to the electrode 14, and (2) connecting the input of the amplifier 29 across the output corners 22 and 24 of the bridge circuit 20. The motor 37 is advancing the slider 32 at a rate determined by the speed control 46 to increase the anodizing voltage from a 'first low voltage. When the voltage on the slider 32 reaches a second voltage, the relay 53 is actuated to close contacts 53a which applies a voltage to one side of the relays 41, 51, 55 and 42.

The relay 55 will remain unactuated for a predetermined time after the relay 53 is actuated. If the resistor 10' reaches a desired value before the relay 55 is actuated, the output of the bridge circuit becomes positive to produce an output signal from the amplifier 29 to initiate conduction through the silicon controlled rectifier 39. If the resistor 10 fails to reach the predetermined value within the time delay period of relay 55 contacts 55a of relay 55 are closed to bypass the rectifier 39. Either the conduction of the rectifier 39 or the actuation of relay 55 actuates the relays 41 and 42.

Contact arm 42b of relay 42 engages contact 420 upon actuation of relay 42 to operate relay 51 which closes a holding contact 51a to hold relay 51 in its actuated posi tion. Contacts 41a, in series with the lead 35, are open by actuation of relay 41 to interrupt the anodizing current. Momentarily, -actuation of the relay 42. also. closes contacts 42a to advance the stepping switch 17 to connect the resistance 11 to the corner 22 of the bridge circuit 20.

Contact arm 51a of relay 51 engages contact 510 to operate relay 58. Operation of relay 58 moves contact arms 58a and 58b in the reversing switch 45 toreverse the operation of the motor 37 to retract the slider 32. After the slider 32 is'retracted to reduce ,the voltage again to the first voltage, the holding current through the relay 53 becomes insufficient to maintain the relay 53 actuated and it then becomes deactuated. Deactuation of relay 53 opens contacts 53a to deactuate relays 41, 42, 51 and 55. Contact arm 51a (FIG. 2) moves from contact 510 to 51b to deactuate relay 58 to operate contact arms 58a and 58b to change the polarity of voltage and connect the advancing. potentials to the .motor 37. Contacts 41a close to apply anodizing voltage to resistor 11 to start another cycle of operation.

It is to be understood that the above-described embodiment is simply illustrative of the principles of the invention and that many other embodiments may be devised without departing from the scope and spirit of the invention.

What is claimed is:

1. A circuit for applying an increasing voltage to an electrolyte contacting a thin film resistor to change the resistor to a desired value of resistance comprising:

means connectable to a power source for supplying a DC. voltage;

a potentiometer having a slider and two end terminals with the end terminals connected across the DC.

voltage supplying means,

reversible motor means for advancing and retracting the slider such that during advancing the voltage on the slider increases from a low voltage and during retracting the voltage on the slider returns to the low voltage;

selectively variable means for determining the rate of advance of the motor means;

resistance measuring means for producing a signal when a resistance equal to or greater than the desired value of resistance is connected to an input thereof; means operable when the motor means isadvancing for alternately connecting (1) the slider across the electrolyte and the resistor; and (2) the resistor to the input of the measuring means; and means operable in response to the signal from the measuring means for preventing the connecting of the slider across the electrolyte and resistor and for reversing the motor means to retract the slider. 2. A circuit as defined in claim 1 which includes a timing means actuated when the voltage on the slider increases to a predetermined voltage for operating said preventing and reversing means if the resistor fails to reach the desired value of resistance within a predeter mined period of time.

3. A circuit for anodizing a plurality of thin film resistors to a desired value comprising:

means for contacting the thin film resistors with electrolyte and responsive to a current through the'electrolyte for anodizing the resistors to increase-their resistance; '7

a bridge circuit means for selectively connecting the resistors between first and second corners of the bridge circuit;

means connectable to a power source for supplying a DC. voltage with respect to a ground terminal;

a potentiometer having a slider and two end terminals with the one end terminal connected to the DC.

voltage supplying means and the other end terminal connected to the ground terminal;

reversible motor means for advancing and retracting the slider such that the voltage on the slider increases from a low voltage when the slider is advancing and returns to the low voltage when the slider retracts;

means for producing a signal when a predetermined polarity of a voltage is applied to input terminals thereof;

means for alternately connecting (1) the slider to the first corner of the bridge circuit and the electrolyte to the ground terminal; and (2) the output terminals of the bridge circuit to the input terminals of the signal producing means; and

means operable in response to the signal from the signal producing means. for preventing the connecting of the slider to the first corner of the bridge circuit, for reversing the motor means to retract the slider, and for operating the bridge circuit means to connect another resistor between the first and second corners of the bridge circuit.

References Cited UNITED STATES PATENTS Hasselman et al. 204228X JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner 

